The present invention relates generally to display devices, and more particularly to display devices that utilize electron-beam excitation of a phosphor, such as cathode ray tubes having multiple color stripes or dots.
In a three-color cathode ray tube (CRT), the traditional phosphors used are (1) zinc-sulfide doped with copper, aluminum and sometimes gold for the green color; (2) zinc-sulfide doped with silver for the blue color; and (3) yttrium-oxysulfide doped with europium for the red color. The zinc-sulfide based green and blue phosphors are both about 20% efficient in light-energy transmission (i.e., conversion of energy from the electron beam to energy illuminated by the excited phosphor), whereas the red phosphors containing yttrium-oxysulfide doped with europium are approximately 11% efficient in light energy.
The phosphors traditionally used in CRT manufacture typically consist of a host crystal and an activator. For example, in the case of traditional CRT red phosphors, some europium atoms are diffused into the yttrium oxysulfide molecular matrix (in percentages typically 6% or lower). Hence, yttrium oxysulfide is known as the xe2x80x9chost crystal,xe2x80x9d while europium is called the xe2x80x9cactivator.xe2x80x9d Each particular phosphor is excited by different forms of energy, in differing concentrations and efficiencies.
As consumers demand increased resolution CRTs, designers have responded by reducing pixel size to increase pixel density. As CRT phosphor display pixel sizes are reduced to increase resolution, the image brightness decreases accordingly. Therefore, there is a need in the art to increase image brightness in CRTs, as well as in other display devices.
FIGS. 1A and 1B are CIE (Commission Internationale d""Eclairage) chromaticity diagrams, which are common ways of representing colors. The CIE diagrams define colors using X and Y coordinates instead of wavelengths or a range of wavelengths of emitted light. All colors that plot in the same location in the color space of the chromaticity diagram will look exactly the same to a standard observer. The perimeter values on the horseshoe curve 101 represent the positions in the chromaticity diagram of all pure colors, i.e., colors with only one wavelength in their spectral distribution. Since all visible colors are made with one or more of these pure colors, all visible colors are inside the region delimited by the curve 101.
The area within triangle 103 represents the potential gamut of colors realizable using conventional P22 red, blue and green phosphors for each pixel of a CRT. The vertices of this triangle are denoted by the primary color used for the display. Any color within the area of triangle 103 can be generated through the use of the three primary color vertices or combinations of the same.
Efforts to improve color CRTs include adding an additional color to the current three color CRT, Referring now to FIG. 2, a CIE is shown in which a blue-green phosphor is added to the red, green and blue phosphors of FIGS. 1A and 1B. This produces a quadrilateral 202 having the same green, red and blue vertices as the three-color displays of FIGS. 1A and 1B, plus a fourth vertex corresponding to the blue-green phosphor. The area bounded by the quadrilateral 202 represents the range of visible colors attainable by combining one or more of the four phosphors. It is seen that the range of visible colors is markedly expanded relative to the tri-color display of FIGS. 1A and 1B. Diagonal 204 is drawn to clearly delineate this expanded color range.
Research regarding suitable cathodoluminescent phosphors for a fourth color has determined that a majority of the possible candidates (e.g., Y2O2S:Pr, Y2O2S:Tb, SrGa2S4:Eu2+, and LaOBr:Tb) exhibit a good chromaticity color point, but also yield a lower light-energy transmission efficiencyxe2x80x94in the realm of approximately 6% or less. Also, a four-color phosphor stripe will be approximately 75% the width of a three-color stripe, while still possessing approximately the same number of phosphor-columns sets. Consequently, a display that utilizes a four-color system will not be as bright as a three-color system. For example, under a monochrome raster, picture brightness will decrease by as much as 25%.
Hence, advancements such as those in connection with high-density displays and four-color displays require corresponding increases in phosphor brightness. Prior improvements in phosphor brightness in color point have been made through phosphor development (e.g., rare-earth phosphors replacing zinc-cadmium phosphors for red color), electron-beam intensity, panel-glass tint, metal-back reflectivity, phosphor-particle packing, phosphor pigments, phosphor particle size, increases in aperture-mask slit size and aperture grill versus shadow mask, black matrix and other milestones. These improvements, however, are generally not sufficient to improve the brightness to the point desired in a four-color cathode ray tube or in a high-density three-color cathode ray tube, without sacrificing device reliability.
As a specific example, increases in phosphor brightness can be achieved through the use of higher power electron guns. Higher power electron guns, however, are more susceptible to high voltage arcing and can also decrease the life expectancy of the phosphor than lower power electron guns. Thus, it is desirable to improve the energy efficiency of the energy conversion from an electron beam to illumination of the excited phosphor.
U.S. Pat. No. 5,821,685 discloses a display with an ultraviolet emitting phosphor. This display uses an electron beam to excite an electron-beam-exciting ultraviolet emitting phosphor, which exclusively excites an ultraviolet-exciting visible-light-emitting phosphor. This two-stage excitation was designed to improve brightness in low-voltage displays and is not sufficient to adequately improve the image brightness in high-voltage displays, such as CRT displays.
The present invention is therefore directed to the problem of increasing image brightness in a cathode ray tube.
The present invention solves these and other problems by providing an additional phosphor-excitation mechanism to improve the light output of the visible-light emitting phosphor. The present invention provides the ability to improve the light output of not only a four-phosphor arrangement, but also for the traditional three-phosphor display or even a two-color or monochrome CRT, such as a black-and-white (black-and-green, black-and-amber, etc.) CRT. In addition, the present invention can improve the light output for CRTs employing more than four colors.
According to one exemplary embodiment of the present invention, one excitation mechanism indirectly excites the visible-light emitting phosphor by first striking a non-visible-light emitting particle (such as an ultraviolet-emitting phosphor) with an electron beam, which then emits non-visible radiation that strikes a visible-light emitting phosphor, thereby activating the visible-light emitting phosphor. A second mechanism simultaneously excites the visible-light emitting phosphor by directly striking the visible-light emitting phosphor with an electron beam. Thus, the same visible-light emitting phosphor is activated by the first indirect mechanism as well as the second direct mechanism.
According to another exemplary embodiment, image brightness can be optimized by disposing non-visible-light emitting particles behind and next to the visible-light emitting phosphors. The result is three different excitation modesxe2x80x94first from a direct activation by the electron beam; second from an indirect activation by non-visible radiation output by the non-visible-light emitting particles disposed behind the visible-light emitting phosphors; and third from an indirect activation by non-visible radiation output by the non-visible-light emitting particles disposed next to the visible-light emitting phosphors.